1. Field of the Invention
The present invention relates to a lubricating device for a four-stroke engine, more specifically relates to a lubricating device for a four-stroke engine that allows lubricating oil to circulate in an engine even if it is tilted in different directions in use.
2. Description of the Related Art
Engines are used as driving sources for working machines, such as a brush cutter or trimmer, and a backpack working machine to be carried by hand or on the back of an operator in use. Generally, engines are classified into two-stroke engines and four-stroke engines, and in recent years, there has been increasing demand for four-stroke engines as driving sources because of increase in public awareness of environmental issues and tightening of emission regulations.
A four-stroke engine needs a larger number of components than a two-stroke engine, and therefore increases in weight. Accordingly, an engine for, in particular, a mobile working machine is required to decrease in weight because an operator works carrying the working machine in necessity.
Thus, with the aim of providing a lightweight four-stroke engine, a lubricating device for a four-stroke engine is disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 2002-147213, which allows lubricating oil to circulate using pressure fluctuation in a crank chamber without providing a lubricating pump separately.
This lubricating device has a first oil passageway provided to penetrate a crank axle that allows an oil tank and a crank chamber to communicate with one another. The lubricating device lubricates around the crank axle by supplying mist oil generated in the oil tank to the crank chamber using a state in which a negative pressure is created in the crank chamber.
Then, the mist oil generated in the oil tank and scattering is fed to driving parts, such as a power transmission mechanism (including an intake valve and an exhaust valve) in a first valve operating chamber and a cam mechanism in a second valve operating chamber arranged above the oil tank when the four-stroke engine stands upright, by using a state in which a positive pressure is created in the crank chamber, so that those driving parts are lubricated.
This lubricating device is configured to suck oil accumulated in the valve operating chamber through a siphon and return the oil to the crank chamber when a piston moves with rotation of the crank axle to create a negative pressure in the crank chamber.
Generally, the concentration of mist oil should be high around the crank axle, and the concentration of the mist oil in the power transmission mechanism and the valve operating mechanism including a cam provided in the valve operating chambers may be lower than the concentration of the mist oil around the crank axle.
However, in the lubricating device disclosed in the above-described patent document, the mist oil generated in the oil tank is supplied to the crank chamber and the valve operating chamber, and therefore the concentration of the mist oil supplied to the valve operating mechanism is substantially the same as the concentration of the mist oil supplied to the crank chamber.
Consequently, there is a technical problem that it is not possible to sufficiently lubricate around the crank axle unless a sufficient amount of mist oil is generated and supplied. In addition, there is another technical problem that if mist oil is supplied too much to the valve operating chamber, an amount of oil remaining in the valve operating chamber increases excessively, and therefore oil is discharged in large quantities while discharging blowby gas to a combustion chamber, so that oil can be consumed fast.
To solve the technical problems involved in the above-described patent document, Japanese Patent Application No. 2009-219044 filed by the applicants of the present application discloses a technique for a lubricating device for a four-stroke engine in which an inner cover having approximately the same shape as a valve operating chamber cover is provided in the valve operating chamber cover, and a gap is formed between the valve operating chamber cover and the inner cover to provide a passageway to collect oil accumulated in the valve operating chamber.
Incidentally, in the four-stroke engine disclosed in Japanese Patent Application No. 2009-219044, a cylinder head and a cylinder block are integrally formed. Here, when the cylinder head and the cylinder block are formed separately, bolt holes to penetrate bolts are required in the cylinder head to mount the cylinder head to the cylinder block.
In this way, bolt holes are required in the cylinder head, and therefore the mounting location and the size of the valve operating chamber cover to be mounted to the cylinder head are limited. Then, these limitations of the mounting location and the size of the valve operating chamber cover limit the size of the inner cover naturally, and therefore may influence valves provided inside the inner cover.